Extractable implantable medical lead

ABSTRACT

A medical electrical lead with a tip-ring assembly optimized to resist damage during extraction. The lead includes an elongated plastic tube and at least two elongated conductors mounted in the plastic tube. A tip-ring assembly is mounted to the distal end of the tube, the tip-ring assembly including a ring electrode coupled to one of the conductors, a tip electrode located distal to the ring electrode and coupled to another of the conductors and two molded plastic components separately fabricated of a plastic harder than the plastic tube, adhered to one another and together defining a circumferential groove in which the ring electrode is located and mechanically coupled to the tip electrode. The tip electrode may be provided with a proximally extending electrode shank covered by a tine sleeve which is fabricated of a plastic softer than the molded plastic components and is adhered to more distally located molded plastic component. The more distally located molded plastic component preferably overlaps the electrode shank to provide a stronger structure.

This application is a continuation of application Ser. No. 09/188,859,filed Nov. 9, 1998, now U.S. Pat. No. 6.052,625.

BACKGROUND OF THE INVENTION

This invention relates generally to medical leads and more particularlyto implantable cardiac leads.

In the context of implantable leads, and particularly in the context ofimplantable cardiac leads, there is often a need to remove a lead afterit has been implanted in a patient's body for some period of time. Inconjunction with lead removal, it is often necessary to apply tractionto the lead, in order to pull it free from tissue adhering thereto. Ithas therefore been recognized for some time that a reinforcement of sometype, extending along the lead body would be beneficial, in order toprevent breakage or partial disassembly of the lead during removal. Forexample, in U.S. Pat. No. 5,231,996 issued to Bardy et al., a variety ofreinforcement mechanisms are disclosed, including cords, filaments,braids, and the like.

More recently, in the context of implantable cardiac leads, the use ofcabled or stranded conductors in place of the previously more commonlyemployed coiled conductors has become more popular. These cabled orstranded conductors, such as disclosed in U.S. Pat. No. 5,584,873 issuedto Shoberg et al., U.S. Pat. No. 5,760,341 issued to Laske et al. andU.S. Pat. No. 5,246.014 issued to Williams et al. inherently provide anincreased tensile strength lead, at least along the segment between thepoint at which the stranded or cabled conductor is coupled to anelectrode and the point at which the conductor is coupled to anelectrical connector at the proximal end of the lead. While this newconductor inherently provides a lead of enhanced tensile strength, inmost transvenous cardiac pacing leads employing cabled or strandedconductors, the conductor which extends to the distal-most portion ofthe lead is still a coiled conductor in order to permit passage of astylet. This distal-most portion of the lead, particularly in thecontext of leads employing tines or other passive fixation mechanisms,is the portion of the lead to be most likely to be firmly embedded infibrous tissue. It is therefore desirable that this portion of the leadin particular should be capable of withstanding high tensile forceswithout breakage.

SUMMARY OF THE INVENTION

The lead disclosed in the present application is particularly designedto reduce problems associated with extraction after implant. In order toaccomplish this goal, the lead is provided with three structuralfeatures, each directed particularly to providing a lead which is easierto extract and less likely to be damaged during the extraction process.

The first feature of the lead is an improved tip-ring assembly,extending from the tip or distal electrode and including the associatedring electrode located proximal thereto. In particular, the tip-ringassembly is adapted for use in conjunction with electrodes employingpassive fixation mechanisms such as tines, in which the tip electrode isfixedly mounted with respect to the lead body, rather than advanceablefrom the lead body as in the context of a screw-in lead. In order toenhance the durability of the lead during the extraction, the tip-ringassembly is fabricated of three molded plastic components, two of whichare fabricated of a relatively rigid plastic, harder than that typicallyemployed in the segment between the tip and ring electrode in bipolarleads employing passive fixation mechanisms. The plastic components areconfigured to provide a mechanical interlock between the tip electrodeand the ring electrode when assembled, and are additionally bonded toplastic insulative tubes or coatings covering coiled and/or cabledconductors extending to the tip-ring assembly area.

In particular, the tip-ring assembly includes a tine sleeve having acentral lumen into which a proximal extending shank portion of the tipelectrode is inserted, a tip-ring spacer component, adapted to be gluedto the proximal end of the tine sleeve and a ring-coil spacer component,adapted to be glued to the tip-ring spacer component, and around which aring electrode may be located. The ring-coil and tip-ring spacercomponents together define a circumferential groove dimensioned toreceive and retain the ring electrode. The distal end of the tip-ringspacer is configured to overlap the proximal end of the electrode shanklocated within the tine sheath, so that an generally rigid assembly isprovided extending from the distal or tip electrode through andincluding the ring electrode of the lead. The ring electrode is coupledto a stranded or cabled conductor which extends to the proximal end ofthe lead, which together with the components of the tip-ring assemblyprovide a first mechanism for transmission of tensile force applied tothe proximal end of the lead all the way to the distal or tip electrode.

A second feature of the invention relates to the provision of insulativecoatings or tubings covering these strand and/or coiled conductorsemployed in the lead which have been treated to enhance their bondingperformance, so that they may usefully be adhered to molded or extrudedplastic components at either end of the lead, further providing for anadditional mechanism of transmission of tensile force along the leadbody. In this context, the conductor coupled to the tip electrode may bea coiled conductor surrounded by a heat shrink tube ofpolytetrafloroethelene (PTFE) which has been treated by etching orotherwise to enhance the ability to bond thereto. The distal end of theheat shrink tube may be bonded adhesively to one or more of the tinesleeve, the ring-coil spacer component and the tip-ring spacer componentand to the connector assembly at the proximal end of the lead. The heatshrink PTFE tubing in conjunction with the associated coiled conductorand the adhesive bonds at the proximal and distal end of the leadprovide a second mechanism for providing enhanced tensile strengthextending along the entire length of the lead. The cabled conductorcoupled to the ring electrode referred to above may correspondingly beprovided with a plastic insulative coating, treated to improve adhesion.For example, the cabled conductor may be provided with a coating ofETFE, modified by plasma coating using silane gas to provide forincreased bonding capabilities. The insulative coating on the cabledconductor may likewise be bonded to plastic components located at theproximal and distal ends of the lead, in turn allowing for distributionof tensile forces between the mechanical joints coupling the cabledconductor to the metal electrode and electrical connector componentslocated at the distal and proximal ends of the leads respectively andadhesive bonds between the insulation and associated nearby plasticparts. The insulation may, for example be bonded to the molded partsassociated with the tip-ring spacer and the connector assembly and/or tothe extruded plastic tubing making up the lead body. By this mechanism,the ability of the cabled conductors to transmit tensile forces from theproximal end of the lead to the distal portion of the lead withoutdamage to the lead is further enhanced. The improved bondingcharacteristics provided by surface treatment of the isulative coatingsand/or tubes also assist in maintaining effective seals against fluidintrusion and migration within the lead body.

A third feature of the lead intended to improve its extractioncharacteristics is directed specifically to leads of the type employingelongated coil electrodes, for example as in implantable cardioversionand defibrillation leads. In some leads of this type, the coil is moldedinto the lead body, such as in U.S. Pat. No. 4,161,952 issued to Kinneyet al. However, a simpler alternative construction mechanism is t simplymount coil electrodes fabricated of single or multifilar coils aroundthe exterior of an extruded tubular lead body. Such coil electrodes aredisclosed in U.S. Pat. No. 4,934,049 issued to Kiekhafer et al., U.S.Pat. No. 5,115,818 issued to Holleman et al. and U.S. Pat. No. 5,676,694issued to Boser et al, all incorporated herein by reference in theirentireties. Experience has shown that discontinuities in lead diameterassociated with the proximal and distal ends of such coil electrodes cancomplicate removal of the lead from its overlying fibrous sheath. Thisis true whether the removal is accomplished by attempting to remove thefibrous sheath prior to extraction lead or whether the lead is to besimply pulled through the fibrous sheath. According to this feature ofthe invention, tubing is provided overlying the extruded lead bodyintermediate the coil electrodes, if there is more than one suchelectrode and intermediate the proximal end of the most proximal coilelectrode and the connector assembly located at the proximal end of thelead. In this fashion, a lead can be provided which is essentiallyisodiametric along the length of the lead body to the distal end of thedistal-most coil electrode, which lead can be fabricated of extrudedmulti-lumen tubing and which does not require molding the coil electrodeinto the lead body. Preferably, if a ring electrode is located distal tothe distal-most coil electrode, it too is configured to be essentiallyisodiametric to the electrode coil and to the lead body or other plasticcomponent separating the distal end of the distal-most coiled conductorand the ring electrode. This particular construction mechanism isespecially convenient in the context of a lead of the type employing anextruded multi-lumen tubular lead body, such as that described in theabove cited patent issued to Shoberg et al. The provision of tubingoverlying the extruded tubular lead body also provides for increasedprotection of the conductors therein without an over-all increase inlead diameter. This use of tubing which is of increased durabilityand/or greater insulative strength further enhances this benefit of thelead body structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a lead according to the present invention,provided with two coil electrodes.

FIG. 2 is a sectional view through the distal portion of the leadillustrated in FIG. 1, illustrating the construction of the tip-ringassembly.

FIG. 3 is a cross-sectional view of the lead of FIG. 1, taken betweenthe coil electrodes mounted thereon.

FIG. 4 is a sectional view through the ring-coil spacer componentillustrated in FIG. 2.

FIG. 5 is a plan view of the distal end of the ring-coil spacercomponent.

FIG. 6 is a plan view of the proximal end of the ring-coil spacercomponent.

FIG. 7 is a sectional view through the tip-ring spacer componentillustrated in FIG. 2.

FIG. 8 is a plan view of the proximal end of the tip-ring spacercomponent.

FIG. 9 is a sectional view of the lead of FIG. 1 in the vicinity of oneof the coil defibrillation electrodes.

FIG. 10 is a cutaway view of a portion of the lead of FIG. 1 adjacentthe connector assemblies.

FIG. 11 is a sectional view through a portion of one of the connectorassemblies of the lead of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a plan view of a lead according to the present invention,embodied as a transvenous cardiac defibrillation lead. The lead isprovided with an elongated lead body 10 which carries four mutuallyinsulated conductors therein, not visible in this view. Three of theinsulated conductors are stranded or cabled conductors, each coupled toone of ring electrode 20, distal coil electrode 24 and proximal coilelectrode 26. A fourth, coiled conductor is coupled to distal or tipelectrode 12. The distal portion of the lead includes the tip-ringassembly which includes the tip or distal electrode 12, the tine sheath16 carrying tines 14, the tip-ring spacer component 18, the ringelectrode 20 and the ring-coil spacer component 22. These componentstogether provide a generally rigid assembly, with the tine sleeve 16fabricated of silicone rubber or relatively softer polyurethanes, andthe tip-ring and ring-tip spacers 18 and 22 are fabricated of relativelyharder plastics, for example polyurethane having a Shore hardness of atleast 75D, to provide a relatively rigid assembly extending to thedistal end of distal defibrillation electrode 24.

At the proximal end of the lead body are three connector assemblies 30,36 and 46, extending from a molded trifurcation sleeve 28, typicallyformed of silicone rubber. Connector assembly 30 carries a singleconnector pin 34, coupled to the conductor coupled to the distal coilelectrode 24, and is provided with sealing rings 32 to seal theconnector assembly 30 within the connector bore of an associatedimplantable cardioverter/defibrillator. Likewise, connector assembly 46is provided with a single connector pin 50 coupled to the conductorcoupled to the proximal coil electrode 26, and is provided with sealingrings 48. Connector assembly 36 takes the form of an IS-1 type connectorassembly provided with a connector pin 44 coupled to the coiledconductor extending to tip electrode 12 and a connector ring 38 coupledto a cabled conductor extending to ring electrode 20. Sealing rings 40and 42 seal the connector assembly within the connector bore of anassociated cardioverter/defibrillator and seal between connector pin 44and connector ring 38. The lead body 10 which extends from thetrifurcation sleeve 28 to the tip-ring assembly at the distal end of thelead is preferably formed of an extruded multi-lumen tube, formed of aplastic substantially less rigid than the ring-tip and tip-ring spacercomponents 18 and 22. Lead body 10 may for example be formed of siliconerubber and/or a relatively softer implantable polyurethane such as thosetypically employed in transvenous cardiac lead bodies. In the areasbetween coil electrodes 24 and 26 and in the area between coil electrode26 and trifurcation sleeve 28, the lead body is provided with an overlaytubing having essentially the same outer diameter as coil electrodes 24and 26, which may also be fabricated of silicone rubber, polyurethane orthe like.

FIG. 2 is a sectional view through the tip ring assembly of the lead ofFIG. 1. At the distal end of the assembly is the distal or tip electrode12 which is provided with an elongated proximally extending shank aroundwhich the tine sleeve 16 is mounted. Electrode 12 may be fabricated ofplatinum/iridium alloy or other biocompatible metal typically used forcardiac pacing electrodes. The shank portion of electrode 12 contains aproximal facing bore in which a monolithic controlled release device 52is located, containing an anti-inflammatory steroid such asdexamethasone compounded into a plastic matrix, for example as disclosedin U.S. Pat. No. 4,972,848 issued to DiDomenico or 4,506,680 issued toStokes, both incorporated herein by reference in their entireties or asimplemented in any of the various commercially available steroid elutingcardiac pacing leads.

The shank portion of the electrode 12 also contains a distally facingbore in which the distal end of coiled conductor 60 is located. Thedistal end of coiled conductor 60 is maintained within the shank bymeans of a crimping or swaging core 56, with conductor 60 compressedbetween the electrode 12 and the crimping or swaging core 56. Crossbores 54 are provided through the distal portion of the shank of theelectrode, allowing for verification of proper placement of coiledconductor 60 during crimping. The distal-most portion of the shank ofthe electrode 12 includes a radially extending, distally facing flange58 which engages with a corresponding internally directed proximallyfacing circumferential flange, molded into tine sleeve 16. Tine sleeve16 is preferably fabricated of silicone rubber or a relatively softerpolyurethane, for example having a Shore hardness of 80A.

Tine sleeve 16 is adhesively bonded to the tip-ring spacer component 18,for example using silicone medical adhesive or a polyurethane basedadhesive, depending on the material of tine sleeve 16. Component 18overlaps the proximal end of the shank of electrode 12 and the proximalend of tine sleeve 16. Component 18 is provided with a proximally facinginternal lumen into which the portion 66 of the ring-coil spacer 22 isinserted. The tip-ring spacer and ring-coil spacer 18 and 22 togetherdefine a circumferential groove with corresponding proximal and distalfacing shoulders which retain ring electrode 20, when assembled.Components 18 and 22 are preferably fabricated of a relatively morerigid plastic than the tine sleeve 16, for example of polyurethanehaving a Shore hardness of 75D.

A length of PTFE tubing 62 is heat shrunk around coiled conductor 60 andat least the distal portion of the outer surface thereof has beentreated to render the tubing bondable, for example by etching by meansof the process commercially available from Zeus Industrial Products,Inc., Orangeburg, S.C. Alternative surface treatments may also beemployed to render the tubing bondable, for example using plasma etchingor adhesion promoters as described in U.S. Pat. No. 4,944,088 issued toDoan et al., incorporated herein by reference in its entirety. Thistubing 62 extends the over the length of the coiled conductor 60 betweenelectrode 12 and connector assembly 36. After assembly, the unfilledspace 64 within the tine sleeve 16 and tip-ring and ring-coil spacers 18and 20 is backfilled with adhesive, bonding the components to themselvesand to the etched PTFE tubing 62 and providing for mechanical interlockof all of these components to provide a generally rigid assemblyextending from the distal electrode 12 to the distal coil electrode 24.

In this view it can be seen that the ring electrode 20 is provided withan inwardly extending lug 70 having a longitudinal bore into which thedistal end of a stranded or cabled conductor 68 has been inserted andwhich is maintained therein by means of crimps applied to the lug 70. Bythis mechanism, and in conjunction with the adhesive and mechanicalinterconnection of the components of the tip-ring assembly shrink tube62, tensile forces applied to the proximal end of the lead aretransmitted to the tip-ring assembly, facilitating removal of the leadwithout breakage or partial disassembly of the distal portion of thelead.

A molded multi-lumen lead body 74 is fabricated of a material softerthan the components 18 and 22, for example extruded silicone rubber, orpolyurethane having a Shore, for example of 80A or 90A, or the like.Lead body 74 is inserted into a proximal facing recess within ring-coilspacer component 22, and is bonded adhesively therein, for example usinga polyurethane or silicone based adhesive. The configuration of the leadbody in cross-section is illustrated in more detail in FIG. 3. Coiledconductor 60 and cabled conductor 68 each extend proximal to theconnector assembly through lumens in extruded lead body 74. Cabledconductor 68 as illustrated is provided with an ETFE coating 72 which isin turn bonded to the interior of the lumen of extruded lead body 74 inwhich it is located. At least the distal outer surface of insulation 72is treated to render it bondable, for example using any of themechanisms discussed above. Alternatively, the coating 72 may be ETFEwhich has been modified by exposure to gas plasma, for example using anapparatus as described in U.S. Pat. No. 5,364,662 issued to DiDomenicoet al, also incorporated herein by reference in its entirety, withsilane used as the feed gas, and ETFE as the plastic to be surfacetreated.

Coil electrode 24 in this view is visible as having essentially the sameouter diameter as the proximal portion of the ring-coil spacer component22, whereby an essentially isodiametric profile is maintained from thetip-ring spacer up to and including the coil electrode 24. As will bediscussed further, this isodiametric profile is maintained proximal tothe illustrated portion of the lead by means of overlay tubing, mountedbetween the electrode coils and between the proximal electrode coil andthe trifurcation sleeve (not illustrated in this Figure).

FIG. 3 illustrates a cross-section through the body of the lead of FIG.1 in an area intermediate the proximal and distal electrode coils. Thelead body 74 is visible in cross-section, and is provided with the totalof six lumens extending therethrough, including a first lumen 82 inwhich the coiled conductor 60, coupled to tip electrode 12 (FIG. 2) islocated. The PTFE tubing 62 surrounding coiled conductor 60 is alsovisible in this view. In second and third lumens 76 and 80 are locatedstranded conductors 84 and 88, each provided with an insulative coating,86, 90 of ETFE. Conductors 84 and 88 couple the proximal and distal coilelectrodes 24 and 26 (FIG. 1) to their associated connector pins at theproximal end of the lead. A fourth lumen 78 carries stranded or cabledconductor 68 which is coupled to ring electrode 20 (FIG. 2). ETFEcoating 72, rendered bondable as discussed above by treatment withsilane gas plasma or otherwise, is also visible surrounding strandedconductor 68. Compression lumens 92 and 94 are provided to enhance theability of the lead to resist crush as described in the above citedShoberg et al patent, and are located diametrically opposite lumens 80and 76.

Stranded conductors 84, 88, and 68 may correspond to those described inthe Shoberg et al., Williams et al. and/or Laske et al. patents citedabove. The number and configuration of the individual strands within theconductor may vary as a function of the expected level of current to becarried by the conductors and as a function of the material of whichthey are fabricated. Typically, it is expected that in the context of apacing/cardioversion/defibrillation lead, the conductors be fabricatedof MP35N alloy wire or silver cored MP35N wire. Coiled conductor 60 maybe a monofilar or multifilar coiled conductor, for example having onethrough five filars, and corresponds to commonly employed coiledconductors used in implantable pacing leads. The coiled conductor 60 maybe also fabricated of MP35N alloy or silver cored MP35N wire.

Surrounding the outer periphery of the lead body 74 is overlay tubing96, which has approximately the same outer diameter and the samethickness as the wire from which the coil electrodes 24 and 26 arefabricated, providing for an essentially isodiametric assembly extendingfrom the proximal coil electrode 26 to the tip-ring assembly illustratedin FIG. 2. A corresponding second overlay tubing extends around leadbody 74 proximal to coil electrode 26 (not visible in this view).

FIG. 4 is a cutaway view through the ring-coil spacer 22. Theorientation of the component in this figure is reversed from that inFIG. 2. The ring-coil spacer component 22 is provided with a throughlumen 100, through which the coiled conductor 60 (FIG. 2). The componentis additionally provided with a generally V-shaped groove 102 in whichthe lug 70 of the ring electrode 20 (FIG. 2) is located. The proximallyfacing end of the component 22 is provided with a recess 110 whichreceives the distal portion of the lead body 74 (FIG. 2). The recess 110is surrounded by a circumferential wall 108 which has an outer diameterisodiametric to that of the distal coil electrode 24. Extendingproximally within recess 110 is a cylindrical sleeve 104 which isinserted into lumen 82 of lead body 74 (FIG. 3). Two proximallyextending pins 106 are also located within recess 110 and are configuredto be inserted into lumens 80 and 76 of lead body 74 (FIG. 3). Bore 112allows for passage of the stranded or cabled conductor 68 from the leadbody 74 into the lug of the ring electrode 20 (FIG. 2). FIG. 5 is a planview of the distal end of the ring-coil spacer 22, illustrating therelationship of the U-shaped groove 102, the bore 112, the through lumen100 and the circumferential wall 108 in more detail. FIG. 6 is a planview of the proximal end of the component 22, illustrating the relativelocations of the circumferential wall 106, the cylindrical sleeve 104,pins 106 and bore 108, in more detail.

FIG. 7 is a sectional view through the tip-ring spacer 18. Again, theorientation of this view is reversed from that illustrated in FIG. 2.The tip-ring spacer 18 is provided with a through lumen 114, throughwhich the coiled conductor 60 (FIG. 2) extends. A distal-facing recess116 receives the proximal end of the tine sleeve 16 (FIG. 2) andoverlaps the proximal portion of the shank of electrode 12 (FIG. 2). Aproximal facing recess 117 receives the distal portion of component 22as illustrated in FIGS. 4-6. A small proximally facing lumen 118 isprovided, which as assembled is aligned with the bore through the lug 70of ring electrode 20 (FIG. 2), providing a recess into which the cabledor stranded conductor 68 may extend. Bores 120 are provided through thesidewall of component 18, allowing for backfilling of the recess 64internal to the tip-ring assembly, as illustrated in FIG. 2. FIG. 8 is aplan view of the proximal end of component 18 and illustrates theconfiguration of the recess 117 which receives the distal portion ofcomponent 22, through lumen 114 and recess 118 in more detail.

FIG. 9 is a sectional view through the lead of FIG. 1 in the vicinity ofthe proximal coil electrode 26. Coil electrode 26 is shown locatedaround lead body 74, flanked on its proximal and distal ends by overlaytubing 96 and 120. Overlay tubing 96 corresponds to the same elementillustrated in FIG. 3 and extends between coil electrodes 24 and 26.Overlay tubing 120 extends to the trifurcation sleeve 28, illustrated inFIG. 1. Together the coil electrodes 24 and 26 in conjunction with theoverlay tubing 96 and 120 provide an essentially isodiametric lead bodyextending from the trifurcation sleeve to the tip-ring assemblyillustrated in FIG. 2. Also visible in this view are stranded or cabledconductor 68 and associated insulative coating 72 and coiled conductor60 and associated heat shrink PTFE tubing 62. Although not illustratedin FIG. 9 it should be understood that the coil electrodes 24 and 26 maybe coupled to stranded or cabled conductors 88 and 84 (FIG. 3) by meansof cross-groove crimp sleeves of the sort described in the above citedpatent issued to Boser et al.

FIG. 10 illustrates a cutaway view through the lead of FIG. 1 in thevicinity of trifurcation sleeve 28. Lead body 74 enters the distal endof trifurcation sleeve 28 and terminates therein. Stranded or cabledconductor 68 extends through lead body 74, out its proximal end andthrough spacer tubing 124 which extends to transition flange 128, whichin turn contains a bore 136 in which the proximal end of cabled orstranded conductor 68 is crimped. At least the proximal outer surface ofETFE insulative coating 72 applied to conductor 68 is made bondableusing one of the methods discussed above and as adhesively bonded to thelumen of lead body 74, in the area adjacent to the point at which itexits lead body 74. This adhesive bond provides for a mechanicalinterconnection between the conductor 68 and the lead body 74, in regionof the trifurcation sleeve, which in turn enhances the ability totransmit tensile force provided by the mechanical and electricalinterconnection of the stranded or cabled conductor 68 to transitionsleeve 128. A coiled conductor 130 is coupled to transition sleeve 128by means of a crimping or swaging core 132. Connector 130 extendsproximally to the IS-1 connector assembly 36 (FIG. 1) where it iscoupled to connector ring 38 in a conventional fashion.

Also visible in this view is PTFE shrink tubing 62 which surrounds thecoiled conductor 60 (FIG. 2). Shrink tubing 62 and conductor 60 extendproximally inside inner tubing 122 which also extends proximally to theIS-1 connector assembly 36. As discussed below, PTFE shrink tubing 62 isadhesively bonded to the interior of inner tubing 122, in the vicinityof IS-1 connector 36, further enhancing the ability of the lad totransmit tensile force from the proximal to the distal tip of the lead.Also visible in this view are two insulative tubes 126 and 134, each ofwhich surrounds one of the stranded conductors coupled to a coilelectrode, and which extend back to the connector assemblies 30 and 46,illustrated in FIG. 1. Tube 134, for example, carries conductor 84 andassociated insulative coating 86. The recess 138 defined withintrifurcation sleeve 28 is backfilled with silicone rubber medicaladhesive, providing a mechanical interconnection of all the componentstherein. This mechanical interconnection also assists in mechanicallycoupling the proximal end of the lead body to IS-1 connector assembly 36and trilurcation sleeve 28.

FIG. 11 is a cutaway view through IS-1 connector assembly 36,illustrating the interconnection of the various components including theconnector ring 38, connector pin 44 and sealing rings 40 and 42. Asillustrated, coiled conductor 60 is coupled to connector pin 44 by meansof a crimping or swaging core 140. Coiled conductor 60 and itsassociated PTFE shrink tubing 62 are located within inner tubing 122,which extends proximally from trifurcation sleeve 28, as illustrated inFIG. 10. Ring electrode 38 is provided with cross bores 142 whichfacilitate backfilling of the recess between the ring electrode 38 andthe inner tubing 122, serving to mechanically interconnect the innertubing 122 to ring electrode 38 and sealing rings 40. Ring electrode 38is in turn mechanically interconnected with connector pin 44 by means ofinjection molded spacer 144, fabricated according to U.S. Pat. No.4,572,605 issued to Hess, incorporated herein by reference in itsentirety. At least the proximal outer surface of PTFE shrink tubing 62applied to conductor 60 is made bondable using one of the methodsdiscussed above and is adhesively bonded to the lumen of inner tubing122, further facilitating transmission of tensile forces from theproximal end to the distal end of the lead body, as discussed above.

The above disclosed embodiment of a lead according to the presentinvention takes the form of a cardioversion/defibrillation lead which isprovided with four electrodes including a tip electrode, a ringelectrode and two defibrillation electrodes and which employs all threeof the described extraction enhancing features in combination.Variations of the invention, using one or more of the featuresenumerated herein particularly adapted to assist in rendering the leadextractable, may of course be used in conjunction with leads having agreater or fewer number of electrodes and conductors. As such, the abovedisclosure should be considered exemplary, rather than limiting, withregard to the claims which follow.

In conjunction with the above specification, we claim:
 1. A medicalelectrical lead comprising: an elongated lead body having proximal anddistal ends and comprising plastic lead components; an electrode mountedto a distal potion of the lead body; an electrical connector mounted toa proximal portion of the lead body; an elongated first conductormounted in said lead body, provided with a covering treated with aninsulative fluoropolymer extending between a first point along theproximal portion of the lead body and a second point along the distalportion of the lead body, being treated to be adhesively bondable atleast adjacent the first and second points and adhesively bonded to theplastic components at the first and second points.
 2. A lead accordingto claim 1 wherein the lead body comprises an elongated plastic tube andwherein the covering is further bonded to the plastic tube at least atone of said first and second points.
 3. A lead according to claim 1 orclaim 2 wherein the conductor is a stranded or cabled conductor andwherein the covering is a coating applied to the conductor.
 4. A leadaccording to claim 1 or claim 2 wherein the lead body comprises moldedplastic parts located at least at one of the first and second points andwherein the covering is additionally bonded to at least one of themolded plastic parts.
 5. A lead according to claim 4 wherein theconductor is a stranded or cabled conductor and wherein the covering isa heat shrink tube enclosing the conductor.
 6. A lead according to claim1 or claim 2, wherein the lead body comprises an elongated plastic tubehaving proximal and distal ends, in which said first conductor ismounted, further comprising: an elongated second conductor mounted insaid plastic tube; a tip-ring assembly mounted to the proximal end ofthe tube, the tip-ring assembly comprising: a ring electrode coupled tothe first conductor; a tip electrode located distal to the ringelectrode and coupled to the second conductor; and two molded plasticcomponents separately fabricated of a plastic harder than the plastictube, adhered to one another and together defining a circumferentialgroove in which the ring electrode is located, a more distally locatedone of the molded plastic components mechanically coupled to the tipelectrode, a more proximal one of the molded plastic componentsadhesively bonded to the plastic tube.
 7. A lead according to claim 6wherein the tip electrode comprises a proximally extending electrodeshank and further comprising a tine sleeve fabricated of a plasticsofter than the molded plastic components and mounted around saidelectrode shank.
 8. A lead according to claim 7 wherein the moredistally located one of the molded plastic components is adhered to thetine sleeve.
 9. A lead according to claim 8 wherein the more distallylocated molded plastic component component overlaps the electrode shank.